In a paper making machine, fabrics are used in each of the forming, pressing, and dryer sections to support the paper web as it is made. In the forming section the stock is deposited onto a moving forming fabric, or between two opposed forming fabrics, to provide a very wet paper web. In the press section, the very wet paper web is carried by at least one press fabric, and subjected to compression in a nip between at least one pair of press section rolls to remove further water from the wet paper web. In the dryer section, the wet paper web is carried by at least one dryer fabric and subjected to heat to remove the remaining water down to a desired level of dryness. The finished paper can then be calendered, and wound onto rolls. In each of these sections, the fabric used, in addition to being subjected to the longitudinal stresses required to keep it reasonably tight and to move it at the desired speed, is also exposed to the conditions prevailing within that section of the papermaking machine. Since the conditions of use in each of these sections are so radically different, fabrics are designed and engineered for each of these sections separately.
The present invention is particularly concerned with multi-layer porous fabrics suitable for use as press section fabrics, although this form of multilayer porous fabric is of more general applicability elsewhere.
The paper web entering the press section will typically contain as much as 85% water. In the press section, much of this water is removed by passing the paper web in contact with one or more press fabrics through at least one nip between at least one pair of opposed press rolls, which applies a significant pressure to the web paper web carried by the press fabric. As the wet paper web is squeezed in the nip, water is transferred from the wet paper web into the press fabrics, and is subsequently removed from the press fabrics by various means. It has become standard practice to use press fabrics having a multi-layer structure, comprising a porous core fabric carrying a layer of nonwoven fibrous batt attached by needle punching to one, or to both, of its sides. The core fabric may be woven or nonwoven. Typical known core fabrics are described by Miller et al in U.S. Pat. No. 4,414,263; by Dufour in U.S. Pat. No. 4,356,225; by Luciano et al in U.S. Pat. No. 4,357,386; by Sutherland et al. in U.S. Pat. No. 4,759,975; by Zehle et al in U.S. Pat. No. 5,277,967, and by Kufferath in U.S. Pat. No. 5,601,691. Many other fabric designs have also been both proposed and used.
The nonwoven fibrous batt part of the press fabric serves various functions. It increases the water carrying capacity of the fabric so as to enable it to transport the maximum amount of water away from the wet paper web. It also serves to reduce any tendency of the core fabric to impart a mark on the wet paper web caused by non-uniform transfer of the compressive loads in a press roll nip.
The core fabric also serves several functions. It supports both the batt and the paper web through the press rolls so that water can be removed from the wet paper web. It also has to accommodate all of the mechanical stresses imposed on the press fabric, which include the required level of tension, and the force required to move the press fabric and the paper web through the press section at the required speed.
Further, a press fabric, comprising the combination of a core fabric and at least one layer of attached batt, must be able to resist the compressive loads imposed as it passes through the press roll nip, or nips, for an acceptable period of time without premature collapse, since collapse severely restricts water carrying capacity.
It can thus be seen that the mechanical properties of the core fabric are often directly related to the performance of a porous fabric of this type.
A disadvantage long associated with the production of many multilayer porous fabrics, which includes press fabrics, is that the method commonly used commercially for attaching one or more layers of nonwoven fibrous batts to one side, or to both sides, of the core fabric is by the needling process. In the needling process, a proportion of the batt fibers are forced into engagement with the core fabric essentially either by forcing these fibers through holes pierced in the fabric by the specially shaped needles, or by pushing a proportion of the batt fibers through the mesh of the fabric. This is a time consuming and costly operation, requiring expensive, specialized equipment. It also suffers from the disadvantages that only a small proportion of the batt fibers become entangled with the core fabric, and that a proportion of the batt fibers are damaged in the needling process. Further, in the needling process, which typically provides from about 500-1,500 needle penetrations per cm.sup.2, the needles pierce holes right through the core fabric which result in an unpredictable level of damage to both the component yarns in a woven core fabric, and to the structure of the core fabric.
Needling can also create fabric defects which must be corrected, which increases production costs. For example, in preparing a press fabric, it is well known that a loop of monofilament, or a broken end of a monofilament, from the core fabric can be pushed out through the batt during needling. If the exposed monofilament is on the paper carrying surface of the press fabric this creates a surface defect which must be corrected as it will cause either marking of the paper, or even holes punctured through the paper. Further, the needles used in the needling loom break regularly. The broken needles have to be replaced, and the remnants of metal left in the fabric being needle punched must be removed. These necessary repairs to the fabric to remove defects, and maintenance of the needling equipment, thus reduce the overall efficiency of manufacture, which impacts significantly on production costs.
Although the needling process is effective in the sense that a useable press section fabric is obtained, it also involves an unknown level of damage to both the batt and the core fabric, which can deleteriously affect the potential useful life of the press fabric.
To overcome these difficulties, it has been proposed to employ a porous fabric without a batt needled thereto as a press fabric, for example as described by Jackson, in U.S. Pat. No. 5,089,324, and in WO93/01350, and by Kufferath in U.S. Pat. No. 4,867,206. Such proposals have met with limited success.
It is therefore desirable that a better option than the prior art needling process for attaching a pre-needled nonwoven fibrous batt to one, or both, of the surfaces of a core fabric should be available. The present invention seeks to provide a multilayer porous fabric which is constructed without using a needling step at all. In the multilayer porous fabrics of this invention a needling step is only used to ensure that the batt used in the applied layer, or layers, is of an acceptable density and internal level of entanglement. Since this needling step is carried out prior to attachment of the batt to the core fabric, the core fabric is not exposed to any damage.